This invention relates to an improvement in the process for modifying the characteristics of starch by subjecting it to heat-moisture treatment, more particularly, to an industrial process for efficiently producing heat-moisture treated starch.
It was reported in detail by V. L. SAIR in "Cereal Chemistry, 44, January, on pages 8 to 26 (1967)" that when potato starch or corn starch is heated at 95.degree. to 100.degree. C. at a relative humidity of 100% the physical characteristics will be modified without change in its granular appearance. In this report, the starch spread as a thin layer about 2 cm thick was put in a pressure vessel of 100% relative humidity and heated at 95.degree. to 100.degree. C. for about 16 hours. It was also attempted that starch adjusted to a moisture content of 18 to 27% is heated in a hot air oven.
As the modifications in physical characteristics of starch caused by heat-moisture treatment, there are known change in equilibrium moisture content, change in X-ray diffraction pattern, change in the swelling power of starch granules, and increase in the gelatinization temperature.
The degree of these changes varies continuously depending upon the temperature and time of heat-moisture treatment as well as the moisture content. In general, potato starch with a high moisture content changes under milder heat-moisture treatment conditions compared with corn starch. A higher degree of changes tends to occur when the moisture content is increased by addition of water.
Particularly, corn starch, which is a material most applicable industrially, has an equilibrium moisture content of about 13% and exhibits a high gelatinization temperature, and hence it has to be treated at a high temperature for a long time (for example, at a temperature of 120.degree. C. or higher for two hours), or it has to be moistened to a moisture content of 20 to 25% prior to heat treatment, in order to effect modification in its physical characteristics by heat-moisture treatment.
In addition, unless a thin starch layer less than 3 cm in thickness is used for heating, modification of same degree is not effected throughout the outer and interior portions. Heat-moisture treatment of starch with a high moisture content obtained by addition of water also causes agglomeration of its granules, thus making it difficult to pulverize the treated products; and heating with steam of 100% relative humidity results in the condensation of steam on the surface of the starch layer, thus agglomerating the gelatinized starch granules, and this also makes it difficult to pulverize the treated products. Consequently, no practical process for industrial production of heat-moisture treated starch has been established and no commercialized product has yet been placed on the market, although preparation of heat-moisture treated starch is not so difficult when carried out in a small amount on a laboratory scale.
This invention relates to a method of processing starch by utilizing the fact that heat-moisture treatment of a material containing starch, such as commercially available corn starch, causes modification of its physical properties, and provides a process for producing heat-moisture treated starch, which is characterized in that the degree of modification can be properly controlled, the treated starch can be easily pulverized without the need for any special operation, and treatment of a large amount can be performed in a short time.